The present invention relates to a method of oxygen delignification of lignocellulosic material, preferably at medium concentration, i.e. between about 8 and 16%.
Since the original introduction of oxygen delignification operating at medium pulp concentrations not much development work has been devoted to this process. Since the use of chlorine free bleaching and the closing of bleach plants have become maters of immediate interest, extended delignification, i.e. the (a) further lowering of the kappa number by means of oxygen has increasingly become more interesting. Extended delignification by oxygen in one or several steps, however, can result in a deterioration in pulp quality. The use of the appropriate conditions, however, can yield several advantages.
It should thus be possible to maintain the yield of the pulp at a higher level than is the case with extended cooking, i.e. cooking to lower the kappa number.
In a multi-step method it should be possible to distribute the chemicals between the steps in order to obtain optimum conditions in every step. Even other conditions could then be optimized.